1. Field of the Invention
The present invention relates to a magnetic recording medium comprising a non-magnetic support and at least one magnetic layer above the support, the magnetic layer comprising a ferromagnetic powder dispersed in a binder.
2. Description of the Related Art
Magnetic recording technology has been widely used in various fields including video, audio, and computer applications, etc. since it has the excellent advantages, which cannot be seen in other recording systems, that the medium can be used repeatedly, a signal is easily converted to electronic form and a system can be built in combination with peripheral equipment, and the signal can easily be corrected.
In general, in order to meet the demand for higher recording density in magnetic recording media for computers, etc., it is necessary to further enhance the electromagnetic conversion characteristics, and it is important to make a ferromagnetic powder finer and make the surface of the medium ultra smooth.
With regard to making a magnetic substance finer, a ferromagnetic metal powder or a ferromagnetic hexagonal ferrite powder of 0.1 μm or less has recently been used as a magnetic substance. Furthermore, in order to highly disperse the fine particle magnetic substance or a fine particulate non-magnetic powder used in a non-magnetic layer when a multi-layer structure in which the non-magnetic lower layer is provided on the surface of a support and a magnetic layer is then provided as an upper layer is employed, there has therefore been proposed a dispersion technique for carrying out planarization by introducing a hydrophilic polar group such as —SO3M (M denotes hydrogen, an alkali metal, or an ammonium salt) into a binder and making a binder chain adsorb on the magnetic substance and the non-magnetic powder via the polar group.
For example, there has been proposed an adhesive composition and a binder of a magnetic recording medium employing a polyurethane resin obtained from an aliphatic isocyanate and a diol having a branched side chain (ref. JP-A-5-70757, JP-A-6-162487, JP-A-8-17036, JP-A-2000-319585, JP-A-2001-126230, JP-A-11-39639, and JP-A-9-69222; JP-A denotes a Japanese unexamined patent application publication). However, even when these binders are used, the dispersibility of the magnetic substance is insufficient; in particular, an ultrafine particle ferromagnetic powder necessary for achieving higher density cannot be dispersed sufficiently, and there is the problem that adequate electromagnetic conversion characteristics cannot be obtained.
Furthermore, when these binders are used, there are the problems that sufficient coating strength cannot be obtained and sufficient repetitive transport durability cannot be guaranteed.